Mjne does that sometimes with different features. Sometimes it won't register my sleep, heartbeat, etc. Try a manual refresh. When you open the app slide you finger down your phone screen from the top just a little bit and it should re-refresh. I have to do that with mine often. If it doesn't help try resetting the device. On the device itself slide finger from top of screen to bottom then keep scrolling till you see the restart and tap it. You won't lose any data.
